Trifocal vs Progressive Lenses: Image Jump, Prismatic Scotoma & Aberration Comparisons

Trifocal lenses combine three distinct optical zones:

• Distance Carrier: Corrected for optical infinity.
• Intermediate Segment (FT-7x28 / FT-8x35): Placed immediately below the pupil, standardized to exactly $50\%$ of the reading addition ($\text{Intermediate Add} = 0.50 \times \text{Reading Add}$).
• Near Reading Segment: Provides full $100\%$ reading addition for $40\ \text{cm}$ focal plane.

When gaze transitions across the segment line, an abrupt vertical prismatic displacement occurs:

$$\Delta_{\text{Jump}} = c_{\text{seg}} \times F_{\text{add}}$$

Where $c_{\text{seg}}$ is the vertical distance (in cm) from the dividing line to the segment optical center. In Flat-Top 28 segments ($c = 0.5\ \text{cm}$) with a $+2.00\ \text{D}$ add, an abrupt $1.00^\Delta$ Base-Down jump occurs, creating an Annular Jump Scotoma that obscures floor edges and stair treads.

Because trifocal segments are spherical surfaces without progressive corridors, Minkwitz's theorem does not apply: unwanted peripheral astigmatism is strictly $0.00\ \text{D}$ across the full horizontal diameter.

However, freeform digital progressives remain superior for modern patients by eliminating visible lines, preventing jump scotomas, and providing continuous focal depth from $35\ \text{cm}$ to infinity.